1. Field of the Invention
The present invention relates to an ink ribbon cassette used in a printer, a typewriter or the like.
2. Description of the Related Art
Several types of ink ribbon cassettes are used in many printers and typewriters. In thermal type printers, one-time disposable ink ribbon cassettes are generally used. In impact dot type printers, one type of ink ribbon cassette includes a relatively long, for example, 10 to 60 meters, endless ink ribbon in the cassette, and another type of ink ribbon cassette includes a relatively short, for example, 0.5 to 1 meters, endless ink ribbon in the cassette. In the latter case, an ink supply means is incorporated in the cassette and the ink ribbon is repeatedly used with the supply of ink thereon.
For example, Japanese Unexamined Patent Publication (Kokai) No. 63-182174 discloses an ink ribbon cassette in which an inking roller having ink absorbed therein and an ink transferring roller is provided in the cassette to supply ink onto the ink ribbon.
Japanese Unexamined Patent Publication (Kokai) No. 1-198379 discloses an ink ribbon cassette in which an inking roller is shown in FIG. 3. Also, FIGS. 1 and 2 of that reference show a pair of gears between which the ink ribbon is pinched and fed. An ink belt is provided in FIG. 1 therein, which is driven by one of the gears to supply ink to the ink ribbon.
Recently, an ink ribbon cassette has become known which comprises a pair of gears between which an ink ribbon 101 is pinched and fed, and ink is transferred from one of the gears to the ink ribbon, as shown in FIG. 7 attached hereto. The upper gear, for example, which transfers ink to the ink ribbon 101, is in contact with an ink supply means having ink absorbed therein, for example, a felt containing ink therein. Therefore, ink absorbed in the felt is scraped off by the upper gear gradually, and transferred to the ink ribbon 101 via the upper gear.
A problem of the above described prior art is that a concentration of ink in the ink ribbon varies due to occasional excessive supply of ink. For example, when the temperature around the felt is relatively high, the viscosity of ink in the felt becomes low and ink flows easily in the felt so that excessive ink is transferred to the ink transferring gear. Also, when the ink ribbon is idle fed and the printing head does not print letters, ink in that portion of the ink ribbon is not consumed and thereafter ink is further supplied by the ink transferring gear, resulting in an excessive ink supply. When excessive ink supply occurs, ink is filled in the spaces between adjacent teeth of the ink transferring gear and a large amount of ink is transferred to the ink ribbon locally to cause a variation of ink concentration in the ink ribbon and thereby deteriorate the quality of the typing.